A variety of power conversion devices capable of providing a variety of types and levels of power for a variety of different purposes are now available on the market. A number of these power conversion devices are designed to output three-phase, alternating current (AC) electrical power for use by three-phase AC machines and other devices. For example, in the field of electric motors and motor drives, a three-phase AC motor can be connected to a motor drive, which includes (and operates as) a power converter and provides three-phase AC electrical power to the motor in a controlled fashion. By controlling the currents (and voltages) applied to a given motor, the motor drive further is capable of controlling motor speed, torque and other motor performance characteristics.
Although power converters can take a variety of forms, many power converters including many of those serving as motor drives are power converters that employ pulse width modulation (PWM) techniques to convert power from one form into another, and to generate the desired three-phase AC output power. Many such PWM power converters include both a rectifier stage and an inverter stage, which are implemented by way of bridges having multiple switching devices such as silicon controlled rectifiers (SCRs), symmetric gate commutated thyristors (SGCTs), integrated gate commutated thyristors (IGCTs), insulated gate bipolar transistors (IGBTs), and a variety of other types of switching devices, depending upon the embodiment. Power converters of this type can include, for example, voltage source inverters (VSIs) and current source inverters (CSIs), among others.
Although many conventional PWM power converters such as the drives mentioned above are highly effective in converting input power into the desired, three-phase AC output power, one aspect of the operation of such PWM power converters that could be improved relates to the manner in which the PWM power converters operate when there are momentary losses of power (e.g., momentary line losses) with respect to the power being input to the power converters. It is well known that, when voltages from a line/utility (or other power source) are reapplied to a drive, transient voltage(s) can be produced due to the resonant nature of the drive's input filter (typically including both capacitors as well as inductors), particularly at the instant at which the voltages are reapplied. Further, if residual voltages remain on the input filter capacitors of the drive when power is reapplied to the drive, the transient voltage(s) experienced by the drive tend to be further exacerbated.
Large transient voltage(s) occurring in a drive can create voltage stress on the capacitors and the semiconductor devices of the drive and potentially result in damage to the drive. Because the presence of residual voltages on the input filter capacitors particularly aggravates the creation of these transient voltages, it is desirable that any input filter capacitors be discharged prior to recommencement of drive operation following an input power lapse. That is, upon the opening of the input terminal(s) of a drive (particularly of its rectifier stage) during power failures, one or more of the input filter capacitors typically are charged, and such charged capacitors should be discharged prior to restarting of the drive/closing of the input terminals. Yet the conventional manner of discharging input filter capacitors in drives, typically by way of the filter capacitors' internal bleeder resistors, is excessively slow (e.g., taking nearly a minute), and is inconsistent with providing a drive that is capable of uninterrupted or substantially uninterrupted operation notwithstanding occasional brief input power lapses.
For at least these reasons, therefore, it would be advantageous if an improved drive or other power converter could be developed that, while employing energy-storage components such as input filter capacitors, also was capable of operating or being operated in a manner that facilitated the rapid discharging of such energy-storage components when the provision of input power to the power converter was disrupted, prior to re-energizing the power converter. It would further be advantageous if such an improved power converter achieving such operation could be realized without the need for many additional structural components.